As is well known, metal powders (which term is used herein to include powders consisting at least in part of one or more metals and/or alloys) can be compacted by feeding them to the intake side of a gap between a pair of rolls. The powder may be compacted at ambient or elevated temperature, and the strip-like product which issues from the exit side of the roll gap may be flat sided, through contoured rolls may be used to provide any desired surface profile on the product.
In all powder rolling applications, a problem is posed by the tendency of powder to spread laterally in the roll gap, i.e., at right angles to the rolling direction. Such a tendency results in the production of strip having weak, low density edges, so that an edge trimming operation becomes necessary. In large scale operations, particularly of the continuous type, it is highly desirable to be able to avoid or to at least to minimize any edge trimming needed since such trimming is not only a labor intensive operation but also represents wasted energy associated with the large amount of scrap produced thereby. The problem of controlling strip edges is particularly acute where the apparatus is designed to produce a strip extending over the entire length of the roll gap. In such a case the lateral spread of powder results in egress of the powder from the roll gap. It is with the production of such strip, i.e., the width of which is equal to the length of the rolls, that the present invention is particularly concerned.
In the past various methods have been suggested for controlling the edge of a strip produced by powder rolling. One type of edge control involves the use of a stationary restraint member urged against the end-faces of the rolls in the vicinity of the roll gap, thereby acting as a closure for the roll nip area. Alternatively, it has been proposed to provide flange-like constructions which are fixed to one of the rolls or integrally constructed therewith so as to overlap the other roll at the extremity thereof or in a groove provided near the extremity thereof. Further alternatives which have been suggested involve the use of one or more rollers mounted with their axes orthogonal to the roll axes and forced against the end-faces of the rolls, or used to urge a strip of metal or rubber into contact with the roll end-faces. Yet another approach which has been advocated involves feeding an edge-restraint strip into the roll gap at both extremities thereof, with the powder being fed between the strips.
None of the above-mentioned approaches has provided an entirely satisfactory solution to the problem. Typical of their shortcomings are:
I. Whenever the edge-restraint device is a stationary member, a static powder zone results in the roll nip region and the strip produced exhibits edges of low density, or even unconsolidated edges;
II. Many of the designs suggested do not adequately prevent powder egress from the roll gap because they make only a tangential contact with the roll end-faces;
III. Devices which employ a moving surface to restrain the strip edge generally cease to operate effectively when wear of that surface takes place. Such wear is inevitable when contact is made with the end-faces of the rolls since different points on these end-faces move, in operation, with different linear velocities;
IV. In the case of apparatus employing narrow bands or belts which contact the roll edges or which are fed between the rolls, the edge control bands may be easily damaged, or they may beome entrapped in the metal strip produced. Moreover, the use of such bands or belts generally restricts the flexibility of the apparatus for producing strips of different thicknesses.